(CNN) - Former U.S. President Bill Clinton landed in North Korea early Tuesday on a mission to negotiate the release of two American journalists imprisoned there since March, according to the country's state news agency and a CNN source.

The North Korean news agency KCNA did not disclose the purpose of the visit in its three-line dispatch. But a source with detailed knowledge of the former president's movements told CNN late Monday that Clinton was going to seek the release of Laura Ling and Euna Lee, both reporters for California-based Current TV - media venture launched by Clinton's former vice president, Al Gore.

The women were arrested while reporting on the border between North Korea and China and sentenced in June to 12 years in prison on charges of entering the country illegally to conduct a smear campaign.